Computer Generated Holograms (CGH) in combination with Fizeau interferometers are widely used as a proven and established tool to inspect aspheric surfaces, cylindrical lenses and free-form surfaces with the highest precision. Aspheres with an additional diffractive structure can also be tested.
DIOPTIC has been offering such CGHs for more than 20 years, with a special feature that still leads to increasing popularity among users: In contrast to conventional CGHs, our so-called DFNL CGHs (Diffractive Fizeau Null Lens) can be attached directly to the interferometer and do not require an additional transmission sphere.
DFNL: compatible with all Fizeau interferometers
DIOPTICs DFNL CGHs are delivered with a standard bayonet mount and can be attached directly to the interferometer. This results in a much easier handling compared to conventional CGHs. The elimination of the transmission sphere also makes it possible to test larger convex surfaces, since the entire entrance aperture of the interferometer can be used.
In contrast to conventional CGHs, DIOPTIC DFNLs are manufactured on precision fused silica substrates with a surface flatness of lambda/10 (PV) or better. Since these substrates also provide the reference surface for the interferometer, a highly accurate reference wavefront is generated. The mechanical stability is guaranteed up to a diameter of 230 mm or 9 inches by appropriate substrate thicknesses of up to 24 mm.
Simple Adjustment, Fast, and Precise Measurements
The simple measurement setup when using DFNL CGHs allows fast and precise inspection of aspherical surfaces, cylinders and free-form surfaces. As a DFNL CGH is attached directly to the Fizeau interferometer, no additional CGH adjustment mechanism is required. Auxiliary holograms to align CGHs to the transmission sphere are also not required. So-called alignment or fiducial holograms, which are individually designed for each tested surface, enable the set-up in the shortest possible time. Free form surfaces in particular can be aligned very well with this method.
The CGH surface, which generates the aspherical wavefront, also acts as a reference surface for the interferometer. With conventional CGHs, transmission wavefront errors of the substrate show up directly in the measurement result. DFNL CGHs do not show this measurement error due to their design. DIOPTIC guarantees a wavefront error of the reference surface of less than λ/10 for all DFNL CGHs. Values smaller than λ/20 can also be achieved on request.
Measurement accuracy below 10nm rms for Diffractive Transmission Spheres (DTS)
For large convex lens surfaces with special radii of curvature, it is often not possible to find suitable transmission sphere interferometer objectives. DIOPTIC therefore regularly produces specifically adapted “Diffractive Transmission Sphere” CGHs (DTS) for testing these surfaces. These DTS CGHs can be calibrated very well using a random ball test. The wavefront measurement shown here has a systematic wavefront error of 6.2nm rms. This is also a value that we typically achieve for testing aspheres.
After calibration using this test, measurement uncertainties of better than λ/50 PV can be achieved with DTS CGHs.
Unfortunately, users of Twyman-Green interferometers cannot use our DFNL CGHs. Of course, in this case we also offer the design and manufacture of conventional CGHs. For quick adjustment of the CGH relative to the lens, we offer a suitable 5-axis CGH adjustment mechanism.
If we have aroused your interest, find out more about aspherical testing in our diffractive optics section.
